Polypropylene resins such as a propylene homopolymer, a propylene-ethylene block copolymer, a propylene-ethylene random copolymer and the like are widely used as general-purpose thermoplastic resins in various molding fields such as film molding, injection molding, extrusion molding, blow molding, extrusion stretching molding, sheet molding and the like, because they are easily available and satisfy, to a certain extent, physical properties required by the market.
In recent years, plastics have been increasingly used in production of industrial parts. For example, it is reported that the amount of plastics used as car parts per one car reaches about 8% by weight based on the weight of the car. In application of plastics as industrial parts, physical properties required for the plastics vary greatly depending on the type of part and the purpose of use. Car parts are usually divided into three groups: functional parts, exterior parts and interior parts. When plastics are used as functional parts, it is required for them to have excellent properties such as stiffness, heat resistance, creep properties and the like. For exterior parts are required to have excellent properties such as impact resistance, appearance, fabrication properties, shape retention, weather resistance and the like. That is, properties required for plastics vary depending on the purpose of use. It is required for interior parts to have properties which are well balanced in properties required for both the functional and exterior parts, taking into account the conditions and state of use. For example, impact resistance required for functional parts is sufficient to be relatively low while on the other hand, the impact resistance required for exterior parts such as a bumper should be high at a low temperature range (0.degree. C. to -50.degree. C.) so that a driver can be protected in a crash. Impact resistance required for interior parts is not so high as required for exterior parts, because the interior parts are used inside the car. That is, it is sufficient for the interior parts to have high impact resistance over a temperature range of up to about -20.degree. C.
With regard to stiffness, polypropylene resins have an average value among thermoplastic resins, such as polyvinyl chloride, polyethylene, rubber-modified polystyrene, an acrylonitrile-butadiene-styrene copolymer (hereinafter referred to as an "ABS resin"), acrylonitrile-styrene copolymer and the like, which have heretofore been used in large amounts as industrial parts. Although polypropylene resins are comparable with ABS resins having high stiffness in respect of stiffness at room temperature, at a high temperature range of about 80.degree. C. at which parts are often used, a reduction in stiffness (high temperature softening) of the polypropylene resins is larger than that of styrene-based resins such as the ABS resins. As a result, the polypropylene resins have problems in that thermal creep, permanent deformation and heat shrinkage of parts made of the polypropylene resins tend to occur. That is, even in a talc-reinforced polypropylene resin, the flexural modulus at room temperature of which is equal to that of an ABS resin, the flexural modulus at 80.degree. C. drops to about one-half of the ABS resin. Moreover, moldings of the polypropylene resins are inferior to those of the ABS resins in appearance, shape retention and fabrication properties. To improve impact resistance of polypropylene resins, a method has been known in which an elastomer such as an ethylene-propylene copolymer rubber (hereinafter referred to as "EPR") and a styrene-butadiene copolymer (hereinafter referred to as "SBR"), or a propylene-ethylene block copolymer is compounded to a propylene homopolymer. A method has also been employed in which among polypropylene resins, a propylene-ethylene block copolymer having excellent impact resistance is used.
In order to improve stiffness and shape retention of molded articles compositions have been developed in which an inorganic filler such as glass fibers, talc and the like is compounded with a propylene homopolymer, or propylene-ethylene copolymers such as a propylene-ethylene block copolymer and a propylene-ethylene random copolymer.
Further, in order to improve stiffness and fabrication properties, many attempts to mix the above-described styrene-based resins with a polypropylene resin have been made. In addition, compositions have been proposed in which to improve compatibility, impact resistance and stiffness of mixtures of polyolefin resins such as a polypropylene-based resin, a polyethylene resin, a poly-4-methylpentene-1 and the like, and styrene-based resins, a block copolymer resulting from a specific hydrogenated styrene-conjugate diene block copolymer is added to the mixtures.
For example, Japanese patent application (OPI) No. 53159/79 (the term "OPI" as used herein means a "published unexamined Japanese patent application") discloses a composition in which styrene-ethylene.multidot.butylene-styrene block copolymer is preliminarily mixed with a polyolefin-based resin and the resulting mixture is mixed with a styrene-based resin.
Japanese Patent Application (OPI) No. 38338/81 discloses a composition in which a hydrogenated block copolymer such as a styrene-ethylene.multidot.butylene-styrene block copolymer is compounded with a resin mixture of a polyolefin-based resin and a styrene-based resin.
In a composition comprising a propylene homopolymer and EPR or a propylene-ethylene block copolymer compounded thereto, a molded article made of the composition is improved in impact resistance, but a problem such as a serious reduction in stiffness occurs and furthermore shape retention (dimensional accuracy and shrinkage) and fabrication properties are not improved.
In a composition in which among polypropylene-based resins, a propylene-ethylene block copolymer having relatively excellent impact resistance is used, a molded article made of the composition is improved to a certain extent in impact resistance, but its stiffness and appearance (luster of the surface of the molding) are seriously reduced and furthermore shape retention (dimensional accuracy and shrinkage) and fabrication properties are not improved at all.
In a composition in which an inorganic filler is compounded with a propylene homopolymer or a propylene-ethylene copolymer, a molded article made of the composition is improved in stiffness, but its impact resistance and appearance (surface luster) are reduced. The percentage of shrinkage of the molded article is improved, but anisotropy occurs, that is, deformation of molded article such as twisting and warpage of the molded article are caused and the shape is undesirably deformed. Moreover, fabrication properties are not improved.
In a composition of a polypropylene-based resin and a styrene-based resin, since compatibility of the two resins are poor, even if they are melt mixed merely mechanically, the resulting mixture is seemingly uniform, but it is brittle and its impact resistance is seriously decreased as compared with the original polypropylene-based resin. That is, a molded article having only a very low practical value can be obtained, and the molded article cannot be used as an industrial part as described above. With regard to fabrication properties, when the composition is molded, phillite-like peeling is developed on the surface of the molded article; that is, no satisfactory fabrication properties have been obtained.
In the composition comprising a polyolefin-based resin and a styrene-ethylene.multidot.butylene-styrene block copolymer as disclosed in Japanese patent application (OPI) No. 53159/79, the operation procedure becomes complicated because preliminary mixing is required, and a molded article made of the composition has a poor impact resistance and, therefore, the composition is unsuitable for use in production of industrial parts as described above.
A molded article made of the composition comprising a polyolefin-based resin and a styrene-based resin as described in Japanese patent application (OPI) No. 38338/81 is not well balanced in impact resistance and stiffness and cannot be used at all in production of industrial parts as described above. For example, when a polyethylene resin is used as polyolefin-based resins, a molded article made of the resulting composition is decreased in stiffness, particularly stiffness at a high temperature range. In the case of the above-described composition in which a propylene homopolymer is used as the polyolefin-based resin, impact strength is seriously decreased although stiffness is effectively improved. In the case of the above-described composition in which the conventional propylene-ethylene block copolymer is used in place of the propylene homopolymer, stiffness at a high temperature range is low although impact resistance is effectively improved. In the case of the above-described composition in which an acrylonitrile-styrene copolymer is used as the styrene-based resin, impact resistance is seriously decreased although stiffness is effectively improved.
As a result of investigations to overcome the above problems of polypropylene-based resin compositions, it has been found that a composition obtained by compounding a specific amount of a styrene homopolymer and a styrene-ethylene.multidot.butylene-styrene block copolymer (a copolymer consisting of polystyrene block, ethylene-butylene random copolymer block and polystyrene block) or a styrene-ethylene.multidot.propylene block copolymer (a copolymer consisting of polystyrene block and ethylene-propylene random copolymer block) to a specific propylene-ethylene block copolymer, when molded, provides a molded article which has excellent impact resistance, stiffness, appearance, shape retention and fabrication properties. The present invention has been attained on the basis of this finding.